Exhaust manifold made of heat-resistant cast steel

ABSTRACT

An exhaust manifold made of heat-resistant cast steel comprising pluralities of flanges each having a hole connected to each exhaust port of a cylinder head of an engine with bolts, pluralities of ports connected to the flanges, and a convergence portion in which the ports are converging, the thickness of the flanges being 80-150% of that of the ports.

FIELD OF THE INVENTION

The present invention relates to an exhaust manifold made ofheat-resistant cast steel, which can be produced with a small number ofsteps at a high yield, and has small weight and excellent thermaldeformation resistance.

BACKGROUND OF THE INVENTION

The exhaust manifold for gathering an exhaust gas from an engine andsending it to an exhaust pipe comprises pluralities of flanges eachhaving a hole connected to each exhaust port of a cylinder head of anengine with bolts, a port connected to each flange, and a convergenceportion in which pluralities of ports are converging. To prevent thethermal deformation of flanges by a high-temperature exhaust gas, theflanges are generally thicker than the ports in the exhaust manifold(FIG. 2) as described in JP 10-26018 A. For instance, the flanges havean average thickness of about 12 mm, and the ports have an averagethickness of about 5 mm. In casting such exhaust manifold, voids arelikely generated in the finally solidified flanges.

As automobile engines are recently required to have increasingly higherperformance and fuel efficiency, an exhaust gas temperature has beenelevating. To secure high-temperature strength and oxidation resistanceat 900° C. or higher, the exhaust manifold is made of heat-resistantcast steel. However, voids are likely to generate because the cast steelsuffers large solidification shrinkage during casting. To cope with thisproblem, each flange is provided with a riser 6 as shown in FIG. 3, sothat a melt is supplied to the flange during solidification. However,risers are not used as part of the products, resulting in a lower yieldper the melt used. Further, risers should be removed after casting, alarger number of steps is needed.

OBJECT OF THE INVENTION

Accordingly, an object of the present invention is to provide an exhaustmanifold made of heat-resistant cast steel, which can be produced with asmall number of steps at a high yield, and has small weight andexcellent thermal deformation resistance.

DISCLOSURE OF THE INVENTION

The exhaust manifold of the present invention made of heat-resistantcast steel comprises pluralities of flanges each having a hole connectedto each exhaust port of a cylinder head of an engine with bolts,pluralities of ports connected to the flanges, and a convergence portionin which the ports are converging, the thickness of the flanges being80-150% of that of the ports.

Hole-surrounding portions are preferably 110-300% as thick as theflanges.

The exhaust manifold preferably has a ridge extending along each portfrom the convergence portion. The thickness of the ridges is 70-140% ofthat of the ports.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is a schematic plan view showing the exhaust manifold of thepresent invention made of heat-resistant cast steel.

FIG. 1( b) is a cross-sectional view taken along the line A-A in FIG. 1(a).

FIG. 2( a) is a schematic plan view showing a conventional exhaustmanifold.

FIG. 2( b) is a cross-sectional view taken along the line B-B in FIG.2(a).

FIG. 3 is a schematic view showing a riser provided for casting aconventional exhaust manifold.

FIG. 4( a) is a schematic plan view showing a preferred example of theexhaust manifolds of the present invention made of heat-resistant caststeel.

FIG. 4( b) is a schematic front view showing another preferred exampleof the exhaust manifolds of the present invention made of heat-resistantcast steel.

FIG. 4( c) is a schematic side view showing a further preferred exampleof the exhaust manifolds of the present invention made of heat-resistantcast steel.

FIG. 5 is a cross-sectional view showing a ridge.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the present invention, the thickness of flanges, ports andbolthole-surrounding portions is expressed by an average value. Thebolthole-surrounding portion in the flange has a thickness t₃ as shownin FIG. 1( b). The ridge has a thickness t₄ in a cross sectionperpendicular to a longitudinal direction as shown in FIG. 5.

As shown in FIG. 1, because the flanges 2 are 80-150% as thick as theports 3, the flanges 2, finally solidified portions, are less likely tohave voids. Accordingly, each flange 2 need not be provided with ariser, making it possible to produce the exhaust manifold with a smallernumber of steps at a higher yield. When the thickness of the flanges 2is less than 80% of that of the ports 3, the flanges 2 are not easilyfilled with a melt, resulting in insufficient melt flow. When theflanges 2 are more than 150% as thick as the ports 3, the flanges 2 arelikely to have voids. The flanges 2 are preferably 85-130% as thick asthe ports 3. Although each flange 2 and the surrounding portion 5 ofeach bolthole 4 after casting and before machining are thicker by apredetermined machining margin 7, the thickness t₁ of the flange 2 andthe thickness t₃ of the surrounding portion 5 of the bolthole 4 areexpressed as thickness from the surface 21 of the flange 2 aftermachining in the present invention, as shown in FIG. 1( b).

Because the thickness of the surrounding portions 5 of the boltholes 4is 110-300% of that of the flanges 2, the flanges 2 are not easilythermally deformed even if the exhaust manifold is exposed to ahigh-temperature exhaust gas. Thus, an exhaust gas is prevented fromflowing out through gaps between the exhaust ports of a cylinder headand the flanges 2 of the exhaust manifold. When the surrounding portions5 of the boltholes 4 are less than 110% as thick as the flanges 2, theflanges 2 are easily thermally deformed by a high-temperature exhaustgas. When it exceeds 300%, voids are likely generated in the flanges 2during casting.

When an ridge 11 extends along each port 3 from the convergence portion8 to each flange 2 as shown in FIG. 4( a), the ports 3 are not thermallydeformed even if exposed to a high-temperature exhaust gas, therebypreventing an exhaust gas from leaking. The thickness t₄ of the ridge 11is preferably 70-140% as thick as the port 3. When t₄ is less than 70%of the thickness of the ports 3, the thermal deformation of the ports 3cannot be sufficiently prevented. When t₄ exceeds 140%, the exhaustmanifold cannot be made light in weight. The thickness t₄ of the ridges11 is preferably 80-120% as thick as the ports 3. Instead of extendingalong each port 3 from the convergence portion 8 to each flange 2 asshown in FIG. 4( a), the ridges 11 may extend from the convergenceportion 8 to the branching portions of the ports 3 as shown in FIGS. 4(b) and 4(c).

To have small weight and excellent thermal deformation resistance, theheat-resistant-cast-steel-made exhaust manifold of the present inventionis preferably made of, for instance, heat-resistant, austenitic caststeel comprising by mass 0.2-1.0% of C, 0.05-0.6% of (C—Nb/8), 2% orless of Si, 2% or less of Mn, 8-20% of Ni, 15-30% of Cr, 0.5-6.0% of Nb,1-6% of W, 0.01-0.3% of N, and 0.01-0.5% of S, the balance being Fe andinevitable impurities.

The present invention will be explained in more detail referring toExamples below without intention of restricting the scope of the presentinvention.

EXAMPLES 1-14 AND COMPARATIVE EXAMPLES 1-3

Exhaust manifolds shown in FIG. 1 were formed by heat-resistant,austenitic cast steel having a composition comprising by mass 0.45% ofC, 1.2% of Si, 1.0% of Mn, 0.015% of P, 0.015% of S, 10% of Ni, 20% ofCr, 1.5% of Nb, and 3.0% of W (Examples 1-8 and Comparative Examples1-3). The thickness t₁ of a flange 2, the thickness t₂ of a port 3, andthe thickness t₃ of an surrounding portion 5 of a bolthole 4 are shownin Table 1. Exhaust manifolds shown in FIG. 4( a) were also formed bythe same heat-resistant, austenitic cast steel as above (Examples 9-14).The thickness t₄ of ridges 11 is shown in Table 1 together with t₁ tot₃.

With respect to each exhaust manifold, a yield (number of steps), andthe thermal deformation of the flanges during use were evaluated asfollows: The results are shown in Table 1.

(1) Evaluation of Yield (Number of Steps)

-   -   Good: Good casting free from voids was conducted without        providing a riser to each flange 2.    -   Poor: Good casting could not be conducted without providing a        riser to each flange 2. Because the risers were used, they had        to be cut after casting.

(2) Each exhaust manifold was connected to exhaust ports of a cylinderhead of a usual engine, to evaluate the thermal deformation of flangeswhen the engine was operated.

-   -   Excellent: Thermal deformation did not occur in both of the        ports 3 and the flanges 2.    -   Good: Thermal deformation did not occur in the flanges 2,        causing no leak of an exhaust gas.    -   Fair: Thermal deformation occurred slightly in the flanges 2,        but there was no leak of an exhaust gas.    -   Poor: Thermal deformation occurred in the flanges 2, causing the        leak of an exhaust gas.

TABLE 1 Thickness (mm) Surrounding Thickness Ratio (%) No. Flange t₁Port t₂ Portion t₃ t₁/t₂ t₃/t₁ Example 1 2.5 3.0 5.0 83 200 Example 23.0 3.0 6.0 100 200 Example 3 3.0 3.0 3.0 100 100 Example 4 3.5 4.0 5.388 151 Example 5 4.0 4.0 8.0 100 200 Example 6 5.0 4.0 12.5 125 250Example 7 5.0 5.0 7.5 100 150 Example 8 6.0 5.0 12.0 120 200 Example 92.5 3.0 5.0 83 200 Example 10 3.0 3.0 6.0 100 200 Example 11 3.0 3.0 3.0100 100 Example 12 4.0 4.0 8.0 100 200 Example 13 5.0 5.0 7.5 100 150Example 14 6.0 5.0 12.0 120 200 Comparative 10.0 4.0 10.0 250 100Example 1 Comparative 2.0 4.0 2.0 50 100 Example 2 Comparative 3.0 5.06.0 60 120 Example 3 Thickness t₄ t₄/t₂ Yield Thermal No. (mm) of Ridge(%) (Number of Steps) Deformation Example 1 — — Good Good Example 2 — —Good Good Example 3 — — Good Fair Example 4 — — Good Good Example 5 — —Good Good Example 6 — — Good Good Example 7 — — Good Good Example 8 — —Good Good Example 9 2.0 80 Good Excellent Example 10 3.0 100 GoodExcellent Example 11 3.6 120 Good Good Example 12 4.0 100 Good ExcellentExample 13 5.0 100 Good Excellent Example 14 6.0 120 Good ExcellentComparative — — Poor Good Example 1 Comparative — — Good Poor Example 2Comparative — — Good Poor Example 3

As is clear from Table 1, the exhaust manifolds of the present inventionhaving flanges 80-150% as thick as ports were produced free from voidswithout providing risers, and suffered only small thermal deformationduring use. Examples 1, 2 and 4-8, in which the bolthole-surroundingportions were 110-300% as thick as the flanges, suffered small thermaldeformation. The exhaust manifolds of Examples 9-14 each having ridgesextending along the ports from the convergence portion suffered smallerthermal deformation.

EFFECT OF THE INVENTION

The exhaust manifold of the present invention made of heat-resistantcast steel and having the above structure, which has small weight andexcellent thermal deformation resistance, can be efficiently producedwith a small number of steps.

1. An exhaust manifold made of heat-resistant cast steel comprisingpluralities of flanges each having a hole connected to each exhaust portof a cylinder head of an engine with bolts, pluralities of portsconnected to said flanges, and a convergence portion in which said portsare converging, the thickness of said flanges being 80-150% of that ofsaid ports.
 2. The exhaust manifold made of heat-resistant cast steelaccording to claim 1, wherein hole-surrounding portions are 110-300% asthick as said flanges.
 3. The exhaust manifold made of heat-resistantcast steel according to claim 1, which has a ridge extending along eachport from said convergence portion.
 4. The exhaust manifold made ofheat-resistant cast steel according to claim 3, wherein the thickness ofsaid ridges is 70-140% of that of said ports.
 5. The exhaust manifoldmade of heat-resistant cast steel according to claim 2, which has aridge extending along each port from said convergence portion.